Support markdown highlighting

[achempion]

Documentation blocks like @doc or @moduledoc expected to contain markdown templates.

I'm wondering, is it possible to integrate elixir-mode with mmm-mode to enable markdown editing for these blocks and describe setup in documentation (or make it enabled by default).

It would allow syntax highlighting, table editing and many more markdown features nicely integrated into working with .ex files.

[jsmestad]

@achempion you could use a similar technique that I used for inline LiveView templates.

https://blog.evalcode.com/phoenix-liveview-inline-syntax-highlighting-for-emacs/

[victorolinasc]

I think this is a candidate for a section "Tips and tricks" in the README. @jsmestad wdyt?

Other than that, I think it is not something this mode should support. Would you all agree?

[victorolinasc]

Added the liveview trick in #505 @jsmestad awesome :)

@achempion would you like to add a snippet for docs?